FIG. 9 is a diagram showing a concept of a cone beam X-ray CT (computed tomography) device in which a plane sensor is used.
In FIG. 9, X-rays from an X-ray source 901 are transmitted through a body 903 to be tested, and the transmitted X-rays are detected by a plane sensor 902 provided at the position opposite to the X-ray source 901. Here, it should be noted that the body 903 to be tested is also called a “subject” 903 hereinafter. Then, while the system is generating the X-rays and detecting the radiated X-rays, the cone beam X-ray CT device executes control so that the X-ray source 901 and the plane sensor 902 together rotate around the subject 903. Alternately, it is possible to constitute the cone beam X-ray CT device so that the X-ray source 901 and the plane sensor 902 are fixed and instead the subject 903 itself rotates.
As above, when one rotation from 0° to 360° is executed, the X-ray source 901 and the plane sensor 902 each stand again in their respective initial positions. Alternately, when the subject 903 rotates from 0° to 360°, the subject 903 faces the initial direction at the initial position.
Incidentally, at that time, the projected image initially acquired by the plane sensor 902 at the angle 0° has to be the same as the projected image acquired after the rotation of 360°.
However, in actual radiography, there is no assurance that a patient who is the subject 903 does not move. That is, if the patient moves during the radiography, the projected image initially acquired by the plane sensor 902 at the angle 0° is consequently different from the projected image acquired after the rotation of 360°. Therefore, if the tomographic image of the subject 903 is created by using such different projected images, a streak artifact appears on the created tomographic image.
To reduce the artifact which appears due to such body movement, in conventional X-ray CT device in which the one-dimensional sensor is used, a method of correcting for deviation or misregistration of the subject occurring due to its body movement by executing interpolation within a range of certain angles is known (for example, see Japanese Patent Application Laid-Open No. H06-114052).
Subsequently, this method will be explained with reference to FIGS. 10A, 10B and 10C. Here, FIGS. 10A to 10C show a method in which the body movement is corrected by using a sinogram in which the channels of the detector are plotted on the axis of abscissa and rotation angles are plotted on the axis of ordinate. Here, it should be noted that each of the curved lines 1001 to 1003 on this sinogram indicates the trajectory of a certain point within the subject 903.
If the subject 903 does not move during the radiography (that is, there is no body movement of the subject 903), as shown in FIG. 10A, a detector channel position A of the point at the time of a start of scan (0°) conforms to a detector channel position B on the point at the time of an end of the scan (360°). On the other hand, if the body movement of the subject 903 occurs, as shown in FIG. 10B, the detector channel position of the point at the time of the end of the scan (360°) which should be essentially the position B shifts to a position B′.
To correct for such deviation or misregistration of the subject 903 occurring due to its body movement, as shown in FIG. 10C, the detector channel position A of the point at the time of the start of the scan (0°) is shifted to a position A′ and the detector channel position B′ on the point at the time of the end of the scan (360°) is likewise shifted to a position B″ both by an amount half as much as a deviation amount between the detector channel position A of the point at the time of the start of scan (0°) and the detector channel position B′ on the point at the time of the end of the scan (360°), so as to conform the position A′ and the position B″ to each other. Then, an interpolation process is executed on the data between the position A′ and the position B″ with respect to all the channels to acquire the data string indicated by the dotted curved line 1003 shown in FIG. 10C.
By using the above method, in conventional X-ray CT device in which the one-dimensional sensor is used, it is possible to reduce the artifact which appears due to the body movement of the subject 903. However, in the cone beam X-ray CT device in which the plane sensor 902 is used, if the above method is applied with respect to each of the horizontal lines constituting the plane sensor 902, there is a problem in that consistency between the adjacent horizontal lines cannot be sufficiently attained, and thus the body movement of the subject cannot be accurately corrected for.
Moreover, if it is judged by using the magnitude of deviation or misregistration of each horizontal line whether or not the body movement of the subject has occured, there is a problem that accurate judgment for the body movement cannot be attained, and consequently there is a concern that body movement may be needlessly corrected.